I had a production line going with all models getting regular (non-upgraded) powertrains

AI made Hybrid Powertrain common, so I added it to my models and made sure the upgrade was purchased on Fit Powertrain slot

When next vehicle arrived at Fit Powertrain the resource comes up as just Powertrain, not Hybrid Powertrain, and no Hybrid Powertrains are ordered

However, inspecting that vehicle after it left Fit Powertrain, it says it has a Hybrid Powertrain installed!

Later, (I assume) when vehicles that started on the line after I added the upgrade to the model reach Fit Powertrain, they show up as needing a Hybrid Powertrain, and the slot starts ordering Hybrid Powertrain resources.

I hope that makes sense. Basically there's a delay where regular Powertrains are being consumed but the vehicle says it's getting a Hybrid Powertrain.

Edit:

If you really want to see something weird, uncheck the Hybrid Powertrain feature on the Fit Powertrain slot. It won't consume any resources (no powertrain at all) but you still get a saleable car. It notes that it's missing the Hybrid Powertrain feature, but it doesn't note that it's missing a Powertrain entirely.